1. Field of the Invention
This invention relates to a method of increasing the radiation hardness of the bird's beak region of a semiconductor device and, more specifically, to an NMOS semiconductor device having implanted impurity extending to the edge of the bird's beak region.
2. Brief Description of the Prior Art
It is well known that most CMOS processes using local oxidation of silicon (LOCOS) for field isolation are not very tolerant to radiation. For example, NMOS transistors processed on the 4/3 FPA LINCMOS process show excessive leakage at less than 20 Krad (Si) total dose. There are three transistor regions that are susceptible to radiation induced leakage, these being the gate or active region, the field isolation region and the moat encroachment or bird's beak region. When a MOS device is exposed to ionizing radiation, electron-hole pairs are generated in the oxide, resulting in oxide trapped charge and interface states. The problem becomes more acute for focal plane array (FPA) readout integrated circuits operating at cryogenic temperatures where little annealing of trap charge takes place. The net result of the above described effect is to cause a negative shift in the gate, field and bird's beak voltage thresholds in NMOS devices. Leakage currents can result from these threshold shifts if the threshold voltage of a thin gate falls to an unacceptably low value or surface inversion occurs in the field oxide or bird's beak region. PMOS transistors rarely show any leakage after irradiation since the negative threshold voltage shifts due to radiation increasing the already negative threshold voltages, making it more difficult to turn the PMOS transistors on.
Radiation results of NMOS subthreshold transistors at cryogenic temperatures have shown that inversion in the bird's beak region is the main reason for NMOS transistor leakage up to about 20 Krad (Si). The bird's beak region has a lower threshold voltage than the field oxide because the oxide is thinner than the field oxide and the channel stop implant used to increase the field threshold voltage does not reach under the nitride or, consequently, under the portion of the bird's beak disposed under the nitride.
There is accordingly a necessity to improve the radiation hardness of the birds's beak region of CMOS semiconductor devices and particularly the NMOS component thereof.